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Old 5th May 2008, 08:54 AM   #1
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Question Testing the transformer for the first time

Hello again

I am about to test the secondary output before connecting the PSU. I
have mounted the transformer on 5mm MDF. Should I connect the mains
earth to the transformer nut?

There is a 3.15A slow blow fuse in the IEC power inlet.

I have also constructed the light bulb tester as per Nuuk's website

Can you suggest any further safety recommendations?

Its quite scary doing this for the first time!

Thanks

Trip
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Old 5th May 2008, 08:55 AM   #2
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Light bulb tester
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Old 5th May 2008, 09:08 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
plug in the light bulb.
Plug in the transformer to the light bulb.
Switch on at the wall socket.
The light bulb should not light nor should it flash, all is OK.

If the bulb lights or glows, then something is wrong.

If the primary is a dual winding type, then one pair of connections is effectively a short across the mains. The light bulb protects against this error and saves the fuse and transformer and you from damage.

If the secondaries are wired incorrectly or burned out then again the bulb protects from further damage.

Each time you add something to your project always use the light bulb for first power up.
It's response tells you a lot about the start up characteristics of the project.

I even wired PSU to Power Amp, +ve to -ve and -ve to +ve with the output catch diodes effectively acting as a dead short across the PSU and the bulb lit. No damage anywhere!!!!

A small modification to the light bulb circuit is worth considering.
Add a push and hold to make switch across the bulb live.
This allows start up through the bulb. If the bulb is out, then check equipment side voltages and temperatures. If all appears OK, then push the extra switch to apply full mains power momentarily to the transformer and check PSU voltage. It should now read final voltage. release switch and move voltmeter to another check point. push to measure again etc.

I asked a similar question about three years ago to find a safe way to start up a dual primary transformer and no-one at that time offered the light bulb solution. It came along some months later & I realised it was what I needed. Things move on.
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Old 5th May 2008, 10:23 AM   #4
Nuuk is offline Nuuk  United Kingdom
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Quote:
Should I connect the mains
No.
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Old 5th May 2008, 10:31 AM   #5
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Quote:
Originally posted by Nuuk


No.

Why not Nuuk? I thought it maybe wise to do so because its the main metallic component in the current setup.
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Old 5th May 2008, 12:04 PM   #6
Nuuk is offline Nuuk  United Kingdom
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Quote:
Why not Nuuk? I thought it maybe wise to do so because its the main metallic component in the current setup.
Well it is only for testing your transformer is set up OK. My philosophy is not to make unnecessary connections that may complicate the issue. If it was your final build, then yes by all means make sure that you have a safety earth connected properly.
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Old 5th May 2008, 12:18 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Nuuk


Well it is only for testing your transformer is set up OK. My philosophy is not to make unnecessary connections that may complicate the issue. If it was your final build, then yes by all means make sure that you have a safety earth connected properly.
Hi Nuuk,
where are the exposed conductive parts for connecting to the Safety Earth?
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Old 5th May 2008, 12:29 PM   #8
Nuuk is offline Nuuk  United Kingdom
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Quote:
Hi Nuuk,
where are the exposed conductive parts for connecting to the Safety Earth?
Hi Andrew,

Well we don't know without seeing the set up, or a picture of it.

But Tripmaster asked specifically about connecting the earth wire to the transformer fixing bolt/washer.
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Old 5th May 2008, 04:36 PM   #9
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With fingers and toes crossed I plugged the light bulb indicator into the transformer and the bulb failed to illuminate. Hooray!

The secondary voltage is reading 27.2vac. Is this reading OK? or a little too high?
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Old 5th May 2008, 05:00 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
now add on the rectifier and smoothing caps.
Measure the voltage with the bulb and then without the bulb. Remember to measure the mains voltage at about the same time.
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